Yeasts frequently undergo mutations during their multiplication and during their use in fermentation, in particular on the industrial scale. However, these yeasts generally retain a normal phenotype because the mutations are often recessive mutations and the industrial strains are normally aneuploid or polyploid (The Alcohol Textbook, Ed Jacques (2003)).
Yeast recycling is often used in alcoholic fermentation. Yeast recycling consists of several successive uses of the yeasts, without re-starting from the initial yeasts or initial strain. Thus, all or part of the yeasts stemming from a fermentation i are used for a subsequent fermentation i+1; then all or part of the yeasts stemming from the fermentation i+1 are used for a fermentation i+2, and so on.
Yeast recycling, sometimes called “serial subculture” in the literature, is a technique which exposes the yeast to a large number of stresses that can lead to mutations in its genetic inheritance and genetic drifts.
In their work, Powell and Diacetis (2007, J. Inst. Brew. 1131(1), 67-74) study the impact of the recycling of brewers' yeasts on their phenotype. At the end of each fermentation, a part of the yeast biomass is taken in order to carry out the subsequent fermentation. Powell and Diacetis observed no modification to the fermentation characteristics of the yeasts during successive subcultures, over approximately 135 generations.
Verma et al. (1983) studied, moreover, the advantage of recycling various Saccharomyces cerevisiae yeasts in the context of ethanol production. After each alcoholic fermentation, the yeasts are subcultured for the purpose of a further fermentation, and the amount of alcohol produced at the end of each fermentation is measured. The authors demonstrated several yeast strains capable of undergoing 6 to 10 successive subcultures without their alcohol production being substantially modified.
Thus, in the literature, yeast stability in terms of alcohol production has therefore been studied in the context of yeast recycling in alcoholic fermentation.
When it is multiplied in a conventional industrial process, the most effective yeast, in terms of alcohol production, known to the applicant company sometimes shows, however, considerable decreases in the amount of alcohol produced. However, industrial-scale yeast multiplication starting from a yeast strain should result in a strain of reliable quality. The term “reliable quality” is herein intended to mean that the amount of alcohol produced by the yeast, in a given alcohol fermentation scheme, is relatively constant.
Moreover, the market for yeast intended for alcohol production is always demanding yeasts that are more effective in terms of amount of alcohol produced.
There is therefore a real need to provide novel yeast strains which give yeasts of reliable quality and which are preferably effective in terms of amount of alcohol produced.